1. Technical Field
The present invention relates to an organic EL device applied to displays or display light sources, and more particularly, to a compound for organic EL desirably applied to a coating type organic EL device and to an organic EL device using the same.
2. Related Art
Recently, the development of organic electro-luminescent devices (organic EL devices) using an organic electroluminescent element (organic EL element) which serves as a self-emission display for replacing a liquid crystal display has accelerated. With respect to such an organic EL device and a method of producing the organic EL device, JP-A-2000-323276 and JP-T-2002-536492 are examples of the related art.
A process where a host material and a light emitting dopant are used as a material of a light emitting layer is applied to a known organic EL device (organic EL apparatus) so as to assure high efficiency of light emission, a change in the color of emitted light, and a long life span. The process is frequently applied to organic EL devices including an organic material provided using deposition, but not to polymer-coated organic EL devices which include a layer formed using an ink jet process (liquid droplet ejecting process) or a spin coating process.
In connection with this, the terms ‘host material and light emitting dopant’ have the following meanings. (1) The host material is capable of providing both holes and electrons.
(2) In the organic EL device that includes the light emitting layer having no light emitting dopant, it is observed that light is emitted from the host material. However, if the light emitting layer has both the light emitting dopant and the host material, it is difficult to observe light emitted from the host material, however, emission of light is observed from the light emitting dopant.
(3) The EL spectrum that is observed for the EL element including both the host material and the light emitting dopant is that of fluorescence or phosphorescence emitted from the luminous center of the light emitting dopant. The luminous center is a portion of the light emitting dopant which is an organic molecular skeleton capable of emitting strong fluorescence/phosphorescence.
The reason (problem) why both the host material and the light emitting dopant are not frequently applied to the polymer-coated organic EL devices is as follows.
(1) When a solution containing the host and the light emitting dopant is applied, bleeding of the light emitting dopant may occur. The reason for this is that, if the solution of materials having a low molecular weight is applied to the polymer and dried, the materials having a low molecular weight flow out from the surface layer during the drying.
(2) The light emitting dopant material has poor hole/electron trapping ability. When the host material is a conjugated polymer, the trapping ability is very poor. In such a conjugated polymer, it is believed that since the holes and the electrons preferentially flow through the host polymer molecule, it is difficult for the light emitting dopant to trap the holes and the electrons.
(3) The development of the light emitting dopant material is time-consuming. The reason for this is that the system used for applying the host and light emitting dopant is inefficient in the polymer-coated organic EL device field in comparison with the deposition type EL (low-molecular-weight EL).